1. Field of the Invention
The present invention relates generally to the speed stage control of an automatic transmission of a vehicle such as an automobile, and more particularly, to a method for a downshifting of an automatic transmission of a vehicle such as an automobile according to a depression of the accelerator pedal, i.e., an increase of throttle opening.
2. Description of the Prior Art
An automatic transmission of a vehicle such as an automobile operates to provide various speed stages generally according to a relation in magnitude between the throttle opening representing the driver's requirement for the power performance of the vehicle and the vehicle speed which responds to the throttle opening in relation to other conditions concerned with running of the vehicle. When the vehicle is running at a relatively high speed with the automatic transmission shifted to a high speed stage (i.e. a speed stage having a low reduction gear ratio), if the accelerator pedal is more depressed at a relatively high rate for more than a certain relatively large amount, the control section of the automatic transmission, particularly an electronic computing section in the modern automatic transmission equipped with an electronic control system, judges a requirement of downshifting of the automatic transmission before the vehicle speed increases in accordance with the increase of engine output power due to the increased depression of the accelerator pedal. It is desirable from the view point of a high controllability of the automatic transmission that the control section responds to the driver's requirement immediately in executing the judged requirement of downshifting. However, if a downshifting from a certain first speed stage to a next lower speed stage is started immediately when the amount of depression of the accelerator pedal has exceeded a value for which the requirement of downshifting for one speed stage is judged, if the depression of the accelerator pedal further proceeds so much that its increase exceeds a second value for which a requirement of downshifting for one more speed stage is judged, a complicated disturbance can occur in the movement of mechanically and hydraulically operating friction engaging means such as clutches and brakes for changing over the speed stages of the transmission.
In consideration of such problems, it has been proposed in Japanese Patent Laid-open Publication 58-30558 to provide a certain predetermined delay time between the moment of judgement of a requirement of downshifting and the execution of the downshifting. In this prior art, the predetermined delay time is a constant time span.
In Japanese Patent Laid-open Publication 62-177346 it has been proposed to provide a delay time between the moment of judgement of a requirement of downshifting from a certain first speed stage to a certain second speed stage, and when a different judgement for a requirement of downshifting from said certain first speed stage to a certain third speed stage different from said second speed stage is made within the delay time, the delay time is changed to be adapted to the downshifting according to the second judgement.
In such an art of taking a delay time between the moment of judgement of a requirement of downshifting and the execution of the downshifting, it is the essential problem how to determine the time span of the delay time. From the view point of quick responsiveness of the automatic transmission it is desired that the delay time is as short as possible, whereas, from the view point of avoiding a virtually useless temporal speed stage shifting of the automatic transmission it is desired that the delay time is relatively long so that the requirement for the speed stage shifting is infallibly confirmed. These two requirements are thus contradictory to one another, and therefore, it is still outstanding for any improvements to provide a better reconciliation between these two requirements.